Daylight and Sunlight Assessments in Harrow

Protecting a neighbour's access to light is an important factor in Harrow planning decisions, and a well-prepared daylight and sunlight assessment can be the difference between a smooth approval and a refusal. Whether you are extending a semi in Pinner or Stanmore, altering a terrace in Wealdstone or South Harrow, or bringing forward a larger residential scheme around Harrow town centre or Rayners Lane, the impact of your proposals on surrounding homes is likely to be examined closely - and it is the applicants responsibility to show that any loss of light is reasonable.

At Sunlight Assessments, we prepare BRE-compliant daylight and sunlight assessments for planning applications across the London Borough of Harrow. Our reports are clear, technically robust, and written with Harrow's own design and amenity tests in mind.

When is an assessment needed in Harrow?

Not every application requires a formal study, but one is commonly expected where:

• Extensions that extend close to a boundary with a neighbouring property, particularly where habitable rooms face the affected elevation
• New residential developments where neighbouring properties may lose access to natural light
• Applications where the Local Planning Authority has identified daylight and sunlight as a material planning consideration
• Side and rear extensions or loft conversions in tightly built Victorian and Edwardian terraces, where neighbouring windows may be particularly close to the proposed works
• Conversions and changes of use where new habitable rooms are proposed

If you are unsure whether your scheme requires an assessment, we are happy to advise on this at no cost.

Harrow's planning policy in detail

Harrow now determines applications against its new Harrow Local Plan 2021-2041, adopted in March 2026, which replaced the borough's earlier Core Strategy and Development Management Policies documents. Read alongside the Mayor's London Plan, it sets the framework for how development across the borough is assessed. Its design and amenity policies require new development to safeguard the living conditions of neighbouring occupiers, including their access to satisfactory daylight, sunlight and outlook, with particular attention paid to the light reaching habitable rooms and kitchens.

Harrow is notable for applying a specific geometric tool to extensions - the council's "45 Degree Code". This is used to gauge whether a proposed extension would cause an unacceptable loss of daylight or outlook to a neighbour, by testing the relationship between the extension and the neighbour's windows against a 45-degree line. Where a proposal fails or is borderline against this code, the council will expect the light impact to be examined more fully, typically through a daylight and sunlight assessment. Read more about the 45 degree rule in our blog.

The detailed householder guidance sits in Harrow's Residential Design Guide SPD, which brings together the council's principles for new homes, conversions and extensions - including the 45 Degree Code and its expectations on amenity, layout and overshadowing. This SPD is the practical reference point for anyone extending or altering a home in the borough, and is read together with the Local Plan.

For the numerical assessment, Harrow relies on the Building Research Establishment's guidance, BR 209: Site layout planning for daylight and sunlight (2022 edition), the nationally recognised benchmark. In effect, an extension in Harrow is often judged both against the borough's 45 Degree Code and, where warranted, the BRE metrics of VSC, NSL and APSH.

You can review Harrow's Local Plan at harrow.gov.uk/planning-developments/local-plan and the Residential Design Guide SPD at harrow.gov.uk/planning-developments/residential-design.

How the assessment works

We use the methodology set out in BRE BR 209 to assess the impact of proposed development on daylight and sunlight. The key tests we carry out include:

Vertical Sky Component (VSC): This measures the amount of sky visible from the centre of a window. BRE guidance recommends that a VSC of 27% or more is generally considered adequate. Where a development results in a VSC that is less than 0.8 times its former value, the reduction may be noticeable to occupants.

No-Sky Line (NSL): This assesses the distribution of daylight within a room by plotting the line beyond which no sky is visible. Where the area of a room that can receive direct skylight is reduced by more than 20%, occupants are likely to notice a deterioration in the quality of daylight.

Annual Probable Sunlight Hours (APSH): This test is used to assess access to sunlight for windows that face within 90° of due south. It considers both the winter and annual totals to determine whether sunlight losses are likely to be material.

Where appropriate, we also carry out sun-on-ground studies to assess the impact on garden or amenity spaces.

What our Harrow report includes

Our daylight and sunlight assessments for Harrow planning applications typically include:

• A clear description of the proposed development and the surrounding context
• Identification of all relevant neighbouring windows and amenity spaces
• VSC and NSL calculations for affected neighbouring properties
• APSH calculations where required
• A comparison of results against BRE BR 209 benchmarks
• A professional assessment of whether any identified impacts are likely to be considered material in planning terms
• Recommendations for design amendments where required, to help reduce impacts and support approval

All reports are produced by qualified consultants with experience of working across the London Borough of Harrow.

Get in touch

If you are planning a development in Harrow and would like to discuss whether you need a daylight and sunlight assessment, please get in touch. We offer a free initial consultation and can turn around a report within 5 working days of receiving the necessary information.